FIG. 32 is a block diagram illustrating an exemplary configuration of an electrophotographic image forming apparatus such as a laser printer or a digital copier. As shown in FIG. 32, a laser beam emitted from a semiconductor laser unit 1001 used as a light source unit is deflected and scanned by a rotating polygon mirror 1002, passes through a scanning lens (fθ lens) 1003, and forms a light spot on a photoconductor 1004, thereby exposing the photoconductor 1004 and forming an electrostatic latent image. A phase-locked loop 1009 synchronizes the phase of a modulation signal generated by a clock generation circuit 1008 with that of an output signal from a photodetector 1005 that detects the laser beam deflected by the polygon mirror 1002. More specifically, the phase-locked loop 1009 generates a phase-synchronized image clock (pixel clock) for each line based on the output signal from the photodetector 1005 and supplies the generated phase-synchronized image clock to an image processing unit 1006 and a laser driving circuit 1007. The laser driving circuit 1007 controls the laser emission time of the semiconductor laser unit 1001 according to image data generated by the image processing unit 1006 and the phase-synchronized image clock generated by the phase-locked loop 1009 for each line, thereby controlling the formation of an electrostatic latent image on the photoconductor 1004.
In recent years, increasing demands for higher printing speed (image forming speed) and higher image quality have been satisfied by increasing the rotating speed of a polygon motor used as a deflector and the frequency of a pixel clock used as a base clock for laser modulation. However, further improvements in printing speed and image quality would be difficult with such conventional methods.
For the above reason, multibeam technologies using plural light sources have been developed to achieve higher printing speed and image quality. A light scanning method using a multibeam technology increases the number of light beams that can be deflected and scanned by a deflector at a time. Such a light scanning method enables decreasing the rotating speed of a polygon motor used as a deflector and the frequency of a pixel clock, thereby making it possible to provide faster and more stable light scanning and image forming.
As a semiconductor laser unit or light sources for providing plural light beams in a multibeam technology, a combination of single-beam laser chips or an LD (laser diode) array having plural light-emitting elements on a single laser chip is being used.
Such a semiconductor laser unit, for example, an LD array, for providing plural light beams is very compact, can perform direct modulation at a very high speed using a driving current, and therefore is being widely used as light sources for a laser printer or the like in recent years. However, since the light output of a semiconductor laser at a driving current fluctuates depending on the temperature, it is difficult to set the light intensity of a semiconductor laser at a specific value. Especially, in a surface emitting laser array having plural light sources on a single chip, since the distances between the light sources are short, temperature changes caused by light emission and extinction and temperature cross-talk cause the amount of light emitted from the light sources to fluctuate.
Patent document 1 discloses a light scanning apparatus having a two-dimensional array of plural light sources that scans a photoconductor by deflecting plural light beams with a deflector. According to an embodiment in patent document 1, the density of light emitting points can be maximized without causing temperature cross-talk between the light emitting points.
Patent document 2 discloses an image forming apparatus using surface emitting lasers. An embodiment in patent document 2 makes it possible to change light intensity of each laser chip pixel-by-pixel and to control the light emission time of each laser chip pixel-by-pixel, thereby making it possible to control the formation of an electrostatic latent image of pixels.
Patent document 3 discloses a light scanning apparatus using surface emitting lasers. An embodiment in patent document 3 obviates the problem of heat cross-talk using a specific arrangement of light sources and thereby makes it possible to form a high-density image.    [Patent document 1] Japanese Patent Application Publication No. 2001-272615    [Patent document 2] Japanese Patent Application Publication No. 2003-72135    [Patent document 3] Japanese Patent Application Publication No. 2001-350111
However, in a conventional light scanning apparatus having plural light sources, since one pixel is normally formed with one light source, it is difficult to correct the position of a pixel with an accuracy higher than the size of the pixel.